The prospect of an America's Cup with just three challengers is growing stronger as the runway to build a monster multihull for the 2013 regatta gets shorter.

Only four teams have begun building the new 72-foot wing-sail catamarans that will duel for sport's oldest trophy in San Francisco in September next year. Emirates Team New Zealand is among those now building the new design, as is the Auld Mug defender, Oracle Racing.

America's Cup Race Management chief executive Iain Murray, in Auckland today for a competitors' forum, remains optimistic that four of the remaining six teams who will sail in the next America's Cup World Series event in Naples will gather the funding necessary to build an AC72. But the second wave of global recession has left some European challengers struggling to keep above water.

"We're hopeful of having more teams outside those who are already building. But for those teams who haven't started yet, the runway is going to run out in the not too distant future as to when they can start [building] to get to the start line," Murray told Boating New Zealand.

It would become "pretty clear pretty quickly" just who will compete in the Louis Vuitton Cup challenger series starting in San Francisco in July 2013, with teams realistically needing to start building a race boat by May this year.

"The bottom line is that it takes eight months to build these boats," he said.

Swedish challenger Artemis has begun work on its first AC72, while the resurrected Italian team, Luna Rossa, is putting together a boat with the help of Emirates Team New Zealand in a unique collaboration. The Italians are now in Auckland sailing against the Kiwi team in the AC45s, smaller versions of the actual America's Cup boat.

Murray doubted that Cup organisers would be able to give any financial assistance to those teams fighting to take their campaigns through to the America's Cup start line. A new challenger was today accepted for the America's Cup World Series warm-up regattas - British Olympic gold medallist Ben Ainslie's team - but has no intention of sailing in the 2013 Cup.

"They're all saying they would like to build an AC72. All have plans afoot where to build, how to build, whose going to do it, with teams on standby, but they need an injection from partners, sponsorship and money to do it," said Murray.

In the meantime, event organisers are working to determine the race schedule for the 2013 regatta, which - like the boats themselves - will have a much different look from traditional America's Cup events.

Teams are discussing the possibility of a best-of-three series each race day, with a winner declared each day. The races are certain to be short and sharp - raced on tight courses and lasting little more than 30 minutes.

With the boats predicted to "go faster than anyone expected" - around 18 knots upwind and 30-35 knots downwind - the races along San Francisco's city -front are likely to be sailed over two-lap courses with three-mile legs.

"The general feeling is that the attention span of people watching is better within 30 minutes. A lot of people watch the start, go away in the middle and come back and watch the finish. Clearly our objective is to keep them in their seat for the whole race," said Australian Murray, who recalls sailing for four and a half hours in 1983 America's Cup races off Newport, Rhode Island.

"For 11 people to sail those boat, you're going to see athletes like the America's Cup has never seen before. These teams are going to be absolutely exhausted after 30 minutes."

Aucklanders may get the chance to see the AC72s in action next summer, with plans to hold informal races in the Southern and Northern Hemispheres.